Fluke DMMs and VFD motor drives

[DaveC]

I'm considering the purchase of a new Fluke DMM. I like the low-impedance
feature of the 117, but the low-pass filter feature of the 87 is also
attractive.

I had a conversation with a support engineer at Fluke today to try to help me
decide between these two meters.

It came down to the question of whether the variable-frequency drives I come
across will have noisy outputs which the low-pass filter will be helpful in
dealing with.

Among other services, I install some (3-phase, mostly) VFDs for 230v motors
on old printing equipment to give them a 2nd life.

I have no idea how many (ie, percentage) of VFDs are "noisy" that will make
measurement difficult with a DMM w/o low-pass filtering. Is this a common
problem?

If you've got some experience in this area I'd like to hear your comments.

Thanks,
Dave

 

[Jon Slaughter]

I'm considering the purchase of a new Fluke DMM. I like the
low-impedance feature of the 117, but the low-pass filter feature of
the 87 is also attractive.

I had a conversation with a support engineer at Fluke today to try to
help me decide between these two meters.

It came down to the question of whether the variable-frequency drives
I come across will have noisy outputs which the low-pass filter will
be helpful in dealing with.

Among other services, I install some (3-phase, mostly) VFDs for 230v
motors on old printing equipment to give them a 2nd life.

I have no idea how many (ie, percentage) of VFDs are "noisy" that
will make measurement difficult with a DMM w/o low-pass filtering. Is
this a common problem?

If you've got some experience in this area I'd like to hear your
comments.

Thanks,
Dave

I would go with the 87 as it seems to be better in just about every way
looking at the specs. It has better accuracy and resolution in almost all
categories.

 

[DaveC]

I would go with the 87 as it seems to be better in just about every way

looking at the specs. It has better accuracy and resolution in almost all
categories.

Accuracy and resolution are great, but for me in my work these really come in
second to features (the low-Z and low-pass features, specifically).

If it were not for the lack of a low-Z feature I'd buy the 87 in a second.

But I need to determine if -- as pertains specifically to VFD voltage
measurement -- the 117 is deficient (ie, does noise really cripple voltage
measurement, and how often is this a problem when working in VFDs) in this
respect.

Thanks.

 

[David L. Jones]

I'm considering the purchase of a new Fluke DMM. I like the
low-impedance feature of the 117, but the low-pass filter feature of
the 87 is also attractive.

I had a conversation with a support engineer at Fluke today to try to
help me decide between these two meters.

It came down to the question of whether the variable-frequency drives
I come across will have noisy outputs which the low-pass filter will
be helpful in dealing with.

Among other services, I install some (3-phase, mostly) VFDs for 230v
motors on old printing equipment to give them a 2nd life.

I have no idea how many (ie, percentage) of VFDs are "noisy" that
will make measurement difficult with a DMM w/o low-pass filtering. Is
this a common problem?

If you've got some experience in this area I'd like to hear your
comments.

Thanks,
Dave

The Fluke 289 has both a low pass filter and LowZ input impedance. But you
get the annoyance of the short battery life and graphical display.
http://us.fluke.com/usen/Products/Fluke+289.htm

Check out the new Fluke 28-II rugged meter, it also has the filter, but no
lowZ:
http://us.fluke.com/usen/Products/Fluke+27+II+28+II.htm

Quote:
"Unique function for accurate voltage and frequency measurements on
adjustable speed motor drives and other electrically noisy equipment (28
II)"
but that is just the normal low pass filter feature.

Dave.

 

[Jamie]

I'm considering the purchase of a new Fluke DMM. I like the low-impedance
feature of the 117, but the low-pass filter feature of the 87 is also
attractive.

I had a conversation with a support engineer at Fluke today to try to help me
decide between these two meters.

It came down to the question of whether the variable-frequency drives I come
across will have noisy outputs which the low-pass filter will be helpful in
dealing with.

Among other services, I install some (3-phase, mostly) VFDs for 230v motors
on old printing equipment to give them a 2nd life.

I have no idea how many (ie, percentage) of VFDs are "noisy" that will make
measurement difficult with a DMM w/o low-pass filtering. Is this a common
problem?

If you've got some experience in this area I'd like to hear your comments.

Thanks,
Dave

A scope meter is the best tool for such a job..

 

[Jon Slaughter]

Accuracy and resolution are great, but for me in my work these really
come in second to features (the low-Z and low-pass features,
specifically).

If it were not for the lack of a low-Z feature I'd buy the 87 in a
second.

But I need to determine if -- as pertains specifically to VFD voltage
measurement -- the 117 is deficient (ie, does noise really cripple
voltage measurement, and how often is this a problem when working in
VFDs) in this respect.

Thanks.

If you really want the low-z and LP filter then why not get the 117 and
create a simple LP model? You could probably even add it to the fluke
somehow. It's pretty simple to do. Either a res and cap(not for current
though).

Essentially the meter probably just averages the digital samples which is
more effective IMO. don't know why they couldn't add that feature to all
there meters.

 

[David L. Jones]

If you really want the low-z and LP filter then why not get the 117
and create a simple LP model? You could probably even add it to the
fluke somehow. It's pretty simple to do. Either a res and cap(not for
current though).

Modding a meter can potentially make it unsafe, so not a good idea to
recommend.
The Fluke 289 has both features, so no need to bodge it.

Dave.

 

[Jamie]

Modding a meter can potentially make it unsafe, so not a good idea to
recommend.
The Fluke 289 has both features, so no need to bodge it.

Dave.

I use my 289 as a clock since I don't have a clock in most places I am
at, except my desk or bench. I can't wear watches.

P.S.
I wish it was a little faster in response.

 

[DaveC]

A scope meter is the best tool for such a job..
[Jamie]

Why?

Also, I already have a hand-held scope (Tektronix) so I don't want to spend
$$ unnecessarily on features I already have in maybe another tool.

Dave

 

[Jamie]

A scope meter is the best tool for such a job..

[Jamie]

Why?

Also, I already have a hand-held scope (Tektronix) so I don't want to spend
$$ unnecessarily on features I already have in maybe another tool.

Dave

Because working with VFD's, especially with Vector mode drives, You see
a lot more happening in the output over what a DMM can show you.

Even my Fluke 289 with it's low pass will give incorrect readings of
what is really happening if a Vector drive isn't tuned, incorrect
induction values, defective encoder, something etc..

If all you're looking for is a ball park figure, then I guess you could
use a low pass DMM.

It's up to you.

For every man, they have their own tool!

 

[DaveC]

The noise issues you mention aren't particularly clear. Many VFDs will

radiate some HF/RF noise, but a well-designed instrument should be
relatively immune to radiated noise. [...]
The peak phase-to-phase potentials should be addressed by Fluke to insure
that the DMM can safely withstand these tests.

I don't think it's a fear that the Fluke won't survive a measurement, but
whether the readings will be accurate -- maybe the noise will confuse the DMM
if not lo-pass filtered.

I would expect that having a handheld, battery powered (floating)
scope-meter instrument would be more appropriate for analysing VFD outputs,
than a DMM would be. After all, it can be determined that a motor drive has an output with common
light bulbs (LERs - light emitting resistors).

LER -- have to remember that one...

You (and others) make a good point that maybe a DMM -- alone -- isn't the
best tool for troubleshooting a VFD. I have a nice handheld Tek scope (222PS)
that might fit the bill nicely (fully isolated; floatable to 850v/ch; rated
to 400v/ch, 800v in differential mode).

I think I'll go for the Fluke 117. It's got low-Z and cap (to 10K uF) mode
that I need. And I'll bring the Tek along when I go to look at a troubled
VFD.

Thanks for your comments. They're were helpful. You just saved me a few
hundred $$.

Dave

 

[Greg O]

I'm considering the purchase of a new Fluke DMM. I like the low-impedance
feature of the 117, but the low-pass filter feature of the 87 is also
attractive.

I had a conversation with a support engineer at Fluke today to try to help
me
decide between these two meters.

It came down to the question of whether the variable-frequency drives I
come
across will have noisy outputs which the low-pass filter will be helpful
in
dealing with.

Among other services, I install some (3-phase, mostly) VFDs for 230v
motors
on old printing equipment to give them a 2nd life.

I have no idea how many (ie, percentage) of VFDs are "noisy" that will
make
measurement difficult with a DMM w/o low-pass filtering. Is this a common
problem?

If you've got some experience in this area I'd like to hear your comments.

Thanks,
Dave

I have been working with VFD's for ~10 years and have never measured the
voltage supplied to the motor. Most VFD's have the capability to show the
output voltage, amps, frequency, so for the most part why would you need to?
Greg

 

[James Sweet]

I would go with the 87 as it seems to be better in just about every way
looking at the specs. It has better accuracy and resolution in almost
all categories.

I love my 87, although I do miss the low ohms feature and audible diode
test that my 79 had.

 

[Jamie]

Owning an 87 is the mark of a True Hardware Guy (tm).

Cheers

Phil Hobbs

Well I must be half way between a hardware guy and a geek!
I use a 289 when not at the bench most of the time for
remote DMM work. I Also like the logging function in it which
seems to work well. Just wish it had a faster sampling on it.